Gas blast circuit breaker with contact closing low pressure responsive safety device

ABSTRACT

A gas blast circuit breaker having a safety device which causes the positive closing of the contacts of the opened circuit breaker according to the normal closing cycle and the locking of the closed contacts when an abnormal pressure drop occurs below the value of the pressure necessary to maintain the contacts in their opened position.

Unite States atent Inventor Appl. No.

Filed Patented Assignee Priority Gerard Billon Grenoble, France Feb. 11, 1970 Nov 23, 1971 Merlin Gerin, Societe Anonyme Rue Henri, Tarze,Grenoble, France Mar. 23, 1967 France Continuation-impart 01'application Ser. No. 708,661, Feb. 27, 1968, now abandoned. Thisapplication Feb. 11, 1970, Ser. No. 10,581

GAS BLAST CIRCUIT BREAKER WITH CONTACT CLOSING LOW PRESSURE RESPONSIVESAFETY DEVICE 6 Claims, 2 Drawing Figs.

US. Cl 200/148 B, 200/82 B, 200/148 R, 317/59 Int. Cl 1l-I0lh 33/54 [50]Field ofSearch 200/148 B, 148 E, 148 D, 148 R, 82 B, 148.8; 317/59;340/242; 251/63; 137/458, 538

[56] References Cited UNITED STATES PATENTS 2,555,898 6/1951 Paterson etal. .1 200/82 B X 2,911,492 11/1959 Beatty 1. 200/148 B X 2,964,60512/1960 Schu1tz.... 200/148 B 3,185,895 5/1965 Easley 317/59 FOREIGNPATENTS 810,763 3/1959 Great Britain ZOO/148.8

Primary Examiner-Robert K. Schaefer Assistant Examiner- Robert A.Vanderhye Auomey-Stevens, Davis, Miller & Mosher ABSTRACT: A gas blastcircuit breaker having a safety device which causes the positive closingof the contacts of the opened circuit breaker according to the normalclosing cycle and the locking of the closed contacts when an abnormalpressure drop occurs below the value of the pressure necessary tomaintain the contacts in their opened position.

' I PATENTEnwuvs um SHEET 1 OF 2 N In INVENTOR GERARD BILLON ATTORNEYSPATENTEDuuv 2 3 l97| SHEET 2 OF 2 Fig.2

INVENTQR GERARD BILLON MM; film/ ATTORNEYS GAS BLAST CIRCUIT BREAKERWITH CONTACT CLOSING LOW PRESSURE RESPONSIVE SAFETY DEVICE The presentapplication is a continuation-in-part of my earlier application Ser. No.708,661 filed Feb. 27, 1968, and now abandoned.

The present invention relates to a gas blast circuit breaker in whichthe arc extinction chambers are continually under pressure both in theopened position and in the closed position of the contacts which arepneumatically held in their opened position by the pressure prevailingin the chambers, a safety device entering into operation in case of anyabnormal drop of the pressure in at least one of the chambers.

If a pressure drop occurs, the pressure may become too low to maintain.the contacts in their opened position and mechanical closing of all ofthe contacts affected by the drop in pressure is then inevitable. Thetime when this closing takes place depends upon the mechanicalproperties particularly of the spring which urges the contacts intotheir closed position, which properties are not always uniform.

This results in a random closing of the contacts affected which resultsin the destruction of the circuit breaker, particularly upon the releaseof a main contact prior to the release of its auxiliary shuntingcontact, or upon the nonsimultaneous closing of elements which areconnected in series or inparallel. This latter eventuality necessarilyoccurs in the case of circuit breakers having a compressed-gas reservoircommon to different zones of contact.

An object of the present invention is to overcome this drawback. Thecircuit breaker in accordance with the present invention ischaracterized by the fact that each pole comprises a plurality ofcontacts and that the safety device causes positive closing of theplurality of contacts of the opened circuit breaker in accordance withthe normal closing cycle and the locking in the closed position of theplurality of contacts of the closed circuit breaker when the pressuredrops below a predetermined value, which value is greater than the valueof the pressure necessary to maintain the contacts in their openposition.

The closing of the circuit breaker then takes place under normalconditions, avoiding any damage to the circuit breaker. Of course, thisaccidental closing may have repercussions downstream of the circuitbreaker, which repercussions would in any event have occurred upon theinevitable later mechanical closing.

According to the present invention, the safety device obtains itsoperating energy from the gas pressure which prevails, for instance, inthe arc extinction chambers, and is capable of operating without anyexternal auxiliary source of energy. There is thus obtained anindependent safety device whose operation does not depend upon anyexternal factor.

The means for accomplishing the foregoing objects and other advantages,which will be apparent to those skilled in the art, are set forth in thefollowing specification and claims, and are illustrated in theaccompanying drawings dealing with several embodiments of the presentinvention. Reference is made now to the drawings in which:

FIG. 1 is a schematic cross section through a pneumatic safety device inaccordance with the invention provided on a pneumatically controlledcircuit breaker having two are extinction chambers electricallyconnected in series;

FIG. 2 shows a device in accordance with the'invention on anoleopneumatically controlled circuit breaker.

In FIG. 1, the gas blast circuit breaker comprises two cylindricalchambers 140 and 140 to defining arcing or arc extinction chambers 152and 152, and second chambers 154 and 154, respectively. The cylindersare mounted by hollow insulators 142 and 142' onto a jointcompressed-gas reservoir 30. Two tubular contacts 144,146 and 144', 146'are placed in arc extinction chambers 1 52 and 152, respectively.Contacts 144 and 144 are stationary, while contacts. 146 ad 146 aremovable and have fixedly mounted thereon drive pistons 148 and 148',respectively. Compression springs 150 and 150' are inserted-in thesecond chambers 154 and 154' between the pistons 148 and 148, and theends of the cylinders and 140' and urge the movable contacts 146 and146' towards the position in which the contacts are closed. Pneumaticrelays 156 and 156' connect pipes 12 and 12, respectively, and in awell-known way to chambers I54 and 154' (see US. Pat. No. 3,185,82 toAmalric issued May 25, 1965, and owned by a common assignee) to causeventing the second chamber behind the drive piston of the movablecontact in response to an opening order and a pressurizing of the secondchamber behind the drive piston in response to a closing order.

More particularly, pressurizing of pipes 12 and 12 urges pistons 160 and160' against compression springs 162 and 162' in a sense causingchambers 154 and 154 to be vented through pipes 164 and 164' toatmosphere at 166 and 166', feed pipes 168 and 168', which communicatewith pressurized chambers 152 and 152, being closed by valve members 170and 170 of relays 156 and 156, which members are secured to pistons 160and 160, respectively.

Through the interior of the hollow pedestal insulators 142 and 142'which communicate freely with reservoir 30, the arc extinction chambers152 and 152' are continually under pressure both in the opened positionand in the closed position of the contacts. This pressure, which acts onthe pistons 148 and 148' in opposition to the springs and 150', issufficient to maintain the contacts in their opened position. It caneasily be seen that any leak which occurs, for instance, in theinsulator 142, will bring about a drop of pressure in the chamber 152,causing the action of the spring 150 to become predominant and therebycausing the closing of the contacts 146 and 144. The chamber 152' isalso affected by this leakage via the tank 30, but it is improbable thatthe contacts 146 and 146' will close simultaneously. This accidental andrandom closing of the contacts may cause the destruction of the circuitbreaker if a chamber is not capable of withstanding the voltage, and theobject of the present invention is to prevent such a closing of thecontacts.

On the other hand, it will now be apparent that depressurization ofpipes 12 and .12 causes pistons and 160' to return with valve membersand 170 to their initial position under the action of springs 162 and162, so that chambers 154 and 154' are pressurized by pipes 68 and 168communicating now with pipes 164 and 164, respectively, to admit thehigh pressure of arcing chambers 152 and 152' in chambers 154 and 154'behind pistons 148 and 148 which cause contacts 146 and 146' to closeunder the action of springs 150 and 150'.

In order to prevent random closing of contacts 146 and 146' in case ofpressure drop in the high-pressure circuit 152, 152; I42, 142'; 30, theinventive circuit breaker comprises a pneumatic control device havingvalve assembly device 158 hereinafter to be described.

The moving unit 8 of the valve assembly 10 comprises valve disc 14cooperating with a stationary seat 16 and a valve disc 18 cooperatingwith a stationary seat 20.

A control piston 22, which slides in a stationary cylinder 24 of thebody of the valve assembly, controls the movement of the moving unit 8of the valve assembly. The pressurization of conduit 26 anddepressurization of conduit 28 causes the displacement of the unit 8towards the right of the valve assembly to open the contacts 146 and146' and the pressurization of the conduit 28 and depressurization ofthe conduit 26 causes the unit 8 to move towards the left and to closethe contacts. The compressed gas reservoir 30 is at the pressure of thearc extinction chambers 152 and 152'. This pressure is communicatedthrough a pipe 32 to the right-hand side of the valve disc 14, as wellas to the left-hand side of a balancing piston 34 which is also rigidlyconnected with the moving unit 8, its right side being subjected to theaction of a compression spring 36 which tends to urge the moving unit 8of the valve assembly- 10 towards the left. The pipes 12 and 12 areconnected as hereinbefore described to the relays 156 and 156' so thatthe placing of the pipe 12 under pressure causes the opening of thecontacts, while the removal of pressure therefrom causes the opening ofthe contacts, while the removal of pressure therefrom causes the closingof the contacts as mentioned above.

This device operates in the following manner:

In the position of the valve assembly shown in the drawing, thecompressed gas is admitted at 28, the conduit 26 being depressurized.The piston 22 is urged towards the left, the valve disc 14 appliesitself to its seat 16 and the valve disc 18 leaves its seat 20 so thatthe pipe 12 is brought into communication with the atmosphere at 38.This condition corresponds to the closed position of the contacts of thecircuit breaker. The active surface of the valve disc 14 is slightlygreater than that of the piston 34 so that the compressed gas comingfrom 32 urges the moving unit slightly towards the left into a positionof stable equilibrium. In order to open the contacts, pressure isadmitted at 26 and pressure is removed from the conduit 28. The pressurewhich acts on the left-hand face of the piston 22 overcomes thedifferential opposing pressure exerted by the gas at 32 so that thevalve disc 14 leaves its seat 16, while the valve disc 18 comes onto itsseat 20. The pipe 12 then communicates with the pipe 32, is pressurizedand the circuit breaker opens. As the active surface of the valve disc18 is slightly less than that of the piston 34, the pressure prevailingin pipe 32 urges the moving unit 8 slightly towards the right andtherefore holds it in position.

In accordance with the invention, a manometric relay 40 communicates viaa pipe 42 with the compressed gas reservoir 30 and via a pipe 44 with achamber 46 of the valve assembly 10, within which chambers the piston 34moves.

The moving unit of the relay 40 comprises a control piston 48 sliding ina fixed cylinder 50 and acted on by a compression spring 52 as well as avalve disc 54 cooperating either with a seat 56, in order to close theopening of the pipe 42, or with an orifice 60 leading to the atmosphere.A calibrated conduit 58 provides communication for the inside of thechamber 45 of the cylinder 50 below the piston 48 wh the space above thevalve disc 54.

The relay 40 and the spring 52 are calibrated in such a manner that whenthe pressure is normal in the reservoir 30 and arcing chambers 152, 152,the moving unit of the relay 40 is in its upper position, the cylinder46 communicating with the atmosphere via the pipe 44 and the orifice 60which is opened by the valve disc 54. The lower face of the piston 48receives the pressure prevailing in pipe 42 via the passage 58, and inthis way the moving unit of the relay 40 remains in its upper position.

An abnormal drop of pressure in the reservoir 30 and therefore in pipe32, in pipe 42 and in the chamber 45 below the piston 48 causes thedescent of the moving unit of the relay 40, at a pressure which isslightly above the pressure at which the contacts would close bythemselves. In this way, the valve disc 54 closes the passage 60 andpermits the space 46 to communicate with the pressure prevailing at 32so that the pressures on the two sides of the piston 34 becomeidentical. As a result, the spring 36 (and possibly the residualpressure prevailing in pipe 32 and acting on the right-hand side of thevalve disc 14 in its right-hand position and on the right-hand face ofthe valve disc 18 in its right-hand position) urges the moving unit 8 ofthe valve assembly towards the left assuming the unit is in theright-hand position or holds and locks said unit in the left-handposition if said unit were in the left-hand position. In the first case,the circuit breaker is closed positively and locked in this closedposition, and in the second case, the circuit breaker is locked in theclosed position, regardless of any opening orders which maybe given andwhich remain inoperative.

In FIG. 2, which shows a safety device in accordance with the inventionon an oleopneumatically controlled device, particularly a gas blastcircuit breaker with several interrupting chambers in series, acompressed gas reservoir 70 feeds and is at the same pressure as the arcextinction chambers. These chambers, as in the embodiment previouslydescribed, are under pressure both for the opened position and for theclosed position of the main contacts, which are held in the openedposition by the pressure in the arc extinction chambers, overcoming anopposing pressure coming, for instance, from closure springs. The gasreservoir 70 communicates with a main reservoir 30 by a pie 210, so thatthe arc extinction chambers, the reservoir 70 and the reservoir 30 areat the same pressure.

The closing and opening of the main contacts is controlled in awell-known manner by a hydraulic transmission via a closing conduit 72and an opening conduit 74 both of which are filled with anoncompressible liquid such as oil.

The closing pipe 72 is connected to valve 180, disposed in thehigh-potential region of the circuit breaker having a highpotentialcompressed gas reservoir 70 and which is similar to the circuit breakershown in FIG. 1 as regards the arcing chamber and their control by meanspistons 148, I48 and pneumatic relays 156 and 156' connected to pipes 12and 12. Piston 182 of valve 180 opens valve member 184 as soon as pipe72 is pressurized and the result is that control piston 186 biased bycompression spring 188 moves to the right as pipe 190 is vented toatmosphere in 192 by valve member 184.

On the other hand, piston 186 is moved to the left against the action ofspring 188 as soon as pipe 74 is pressurized (and pipe 72depressurized), piston 194 of valve 196 connecting pipe 198 to pipe 200and thus to reservoir 70 by lifting of valve member 202 causingpressurizing of piston 186.

The movement of piston 186 is communicated to valve members 18 and 14 ofvalve assembly 10 which is identical to valve assembly 10 of FIG. 1 asfar as concerns the part controlling pressurizing and depressurizing ofpipe 12.

The hydraulic closing control is obtained by an oil-pressure transmitter76 which comprises an oil-filled cylinder in communication with theconduit 72 in which a piston 78 can slide to cause the displacement ofthe column of liquid in the conduit 72. This piston 78 is rigidlyconnected with a pneumatic control piston 80 which moves in a cylinderand is subject on one of its faces to the action ofa return spring 82and on the other face, to the pressure prevailing in a chamber 84 of thecylinder.

The chamber 84 is connected by a conduit 86 to a threeway valve 88 withan electromagnetic control 90 which places the conduit 86 incommunication either with the atmosphere via a passage 94 or with thecompressed-gas reservoir 30 via a conduit 92.

The opening control device is identical to the closing device andtherefore will not be described in detail. The same reference numbers,provided with a prime sign, have been used in FIG. 2 to designatesimilar parts. The three-way tripping valve 88' places the chamber 84'either in communication with the atmosphere or under the pressure of thereservoir 30, which causes the displacement of the column of oil in theconduit 74 and an order for the opening of the main contacts.

A limit relay 96 comprises a control piston 98, one of whose faces issubject to the pressure prevailing in the conduit 86 via a pipe 100,while the other face is subject to the pressure prevailing in theconduit 86 via a pipe 102, as well as to the action ofa compressionspring 104. The piston 98 is rigidly connected with a three-way valve106 balanced by a diaphragm 108 which places a pipe 110 in communicationeither with the atmosphere via a passage 114 or with the reservoir 30via a conduit 112. The pipe 110 is connected via a delay device 116which delays flow in one direction to an end-of-stroke detector 118,known per se, which can control the control or locking contacts 123.

The operation of this known device is clear and will be described onlybriefly: an electric engagement order imparted to the electromagnet 90actuates the three-way valve 88 so as to admit the pressure from thereservoir 30 via the conduits 92 and 86 into the chamber 84. Thispressure acts on the piston 80 and moves it upward, against the actionof the spring 82, to drive the piston 78 thereby displacing the liquidin the conduit 72 and lifting the valve member 184. The control piston186 moves to the right and 48 pipes 12 and 12' are depressurized tobring about the closing of the contacts.

The pressure of the reservoir 30 will have simultaneously been admittedinto the pipe 102 which, acting on the piston 98 of the limit relay 96,brings the three-way valve thereof into the left-hand position whichcorresponds to the venting of the pipe 110 to the atmosphere. The limitrelay 96 is adapted to remain in this position under the action of thespring 104 and of the balancing diaphragm 108.

The end-of-stroke detector 118 is brought by its return spring 119 intothe position shown in FIG. 2, the compressed gas escaping through thedelay device 116 and the pipe 110 to the atmosphere.

A tripping order given to the electromagnet 90 causes in a similarmanner the displacement of the column of fluid in the conduit 74 andthereby, in mentioned manner, an opening of the main contacts. At thesame time, due to the fact that the pipe 100 is placed under pressure,the piston 98 of the'limit relay 96 is urged towards the right, causingthe pipe 110 to be placed in communication with the pressure reservoir30. It is easy to see that this position is also a stable position thepressure of the compressed gas then acting on the entire surface of thevalve 106 against the action of the return spring 104.

The placing of the pipe 110 under pressure actuates the end-of-strokedetector 118. The safety device, according to the invention, comprises amanometric valve 120, one control piston 122 of which is subjected onone of its faces to the pressure prevailing in the reservoir 30 via aconduit 124. This piston- 122 is rigidly connected with a three-wayvalve 126, which in one position corresponding to the upper positionshown in FIG. 2, places a conduit 128 in communication with the pipe 110via a conduit 130 and in the other position, the bottom position, placesthe conduit 128 in communication with the atmosphere through a passage129.

The manometric valve 120 is of such a nature that the resultant of theforces acting on the moving unit, due to the active surfaces subjectedto the pressure and possibly to the spring 121 acting on it, maintainsthe latter in the bottom position as long; as the pressure in theconduit 124, which is equal to that of the reservoir 30, is not lessthan a predetermined critical value. This critical value is aspreviously, greater than the minimum pressure causing an automaticclosing of the contacts.

The conduit 128 feeds a pneumomechanical converter 132 which may be asimple pneumatic actuator whose rod 134 cooperates with the moving unitof the three-way valve 88, the assembly being arranged in such a mannerthat in case the conduit 128 is placed under pressure, the rod 134positively pushes the three-way valve 88 into the position in which thepressure is admitted into the chamber 84 of the oil-pressure transmitter76.

The safety device functions as follows:

As long as the pressure in the reservoir 30 and therefore in the conduit124 is greater than the critical value, the manometrievalve 120 will bein its bottom position which places the pneumomechanical converter 132in communication with the atmosphere so that the latter will thereforenot participate in any manner in the operation of the control device.

In case of an accidental drop of pressure in the reservoir 30 to belowthe critical value, the moving unit of the manometric valve 120 movesupward, as described above, closing off the communication with theatmosphere of the conduit 128 and placing the latter in communication,via the conduit 130, with the pipe 110.

If the circuit breaker or circuit breakers are in the closed position,the limit relay 96 is in the position shown in FIG. 2, in which the pipe110 communicates with the atmosphere through the passage 114.

The actuation of the manometric valve 120 will therefore have the soleeffect that the communication with the atmosphere is established by thepassage 114, and no longer by the passage 129, which is withoutimportance.

If, on the other hand, the circuit breaker or circuit breakers are inthe opened position, the limit relay 96 is necessarily in the otherposition in which the pipe 110 is under pressure. The

gas under pressure will feed the actuator 132 via the conduits 128, 110,112, from the reservoir 30 and, under the action of said pressure, therod 134 will actuate the valve 88 which causes the positive closing ofthe contacts.

It should be noted that upon placing in operation after completedischarge of the pressure, for instance, after a checkup or repair ofthe apparatus, the actuator 132 will not bring about an unintendedclosing command. In case of a lack of pressure in the reservoir 30, thelimit relay 96 is, as a matter of fact, automatically brought back intothe position shown in FIG. 2 by the spring 104, thus venting the conduit130 to the atmosphere. The increase in pressure in the conduit 112 canin no way cause a displacement of the three-way valve 106 and anyoperation of the actuator 132 is therefore excluded.

It should be noted that the safety devices of FIGS. 1 and 2 do notrequire any external source of energy and thus assure great reliabilityin operation.

Of course, the invention is by no means limited to such independentdevices, and the pneumatic safety device in FIG. 2 can, for instance, besupplemented or replaced by a pneumatic-electric safety device (notshown), in which a pressure contact subject tothe pressure of thecompressed gas in the reservoir is actuated in case the pressure dropsbelow the critical .value, and causes the energizing of the solenoidclosing valve 88.

It is obvious that certain pneumatic systems described can be replacedby equivalent mechanical, hydraulic or electrical systems, and, viceversa, in a manner well known to those skilled in the art. The device ofthe invention can also be applied to other types of circuit breakercontrols.

In the embodiments more particularly described, the circuit breakercomprises a plurality of elements in series whose contacts are closedsimultaneously. The same problem of damage to the circuit breaker arisesin the case of a circuit breaker having a plurality of chambers inparallel or a single are extinction chamber per pole, but havingauxiliary contacts which must be actuated in a predetermined order, andthe device in accordance with the invention provides a solution to theseproblems.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope ofthe invention being indicated by theappended claims rather than the foregoing description, and all changeswhich come within the meaning and range of equivalency of the claims aretherefore to be embraced therein.

lclaim:

1. A gas blast circuit breaker comprising at least one arcing chambercontaining at least one pair of separable contact members, means topressurize said arcing chamber in the open-"circuit and in theclosed-circuit position of said contact members, first biasing meansbiasing said contact members in their open-circuit position towardstheir closed-circuit position, second biasing means responsive to thepressure in said arcing chamber biasing said contact members continuallytoward their open-circuit position, nonnal service control means toselectively effect the opening and the closing of said contact members,whereby in the open-circuit position of said contact members said secondbiasing means override said first biasing 'means and maintain saidcontact member in their open-circuit position only when said pressure ishigher than a first predetermined value, emergency means responsive tothe pressure in said arcing chamber, said emergency means feeding-apositive closing order into said normal service control means to effectclosing of said contact members when in their open-circuit position andmaintaining in the closed-circuit position of the closed contact memberswhen said pressure in said arcing chamber drops below a secondpredetermined value, said second predetermined value being higher thansaid first predetermined value.

2. A gas blast circuit breaker as set forth in claim 1 comprising, foreach pole, a plurality of series-connected pairs of contact members,wherein said emergency means effect the closing of all the pairs ofcontact members from the open-circuit position when said pressure dropsbelow said second predetermined valve.

3. A gas blast circuit breaker as set forth in claim 1, wherein saidnormal service control means effect a pneumatic control of the movementof said contact members, said emergency means comprising apressure-sensitive valve feeding a pneumatic closing order into saidnormal service control means when said pressure drops below said secondpredetermined value.

4. A gas blast circuit breaker as set forth in claim 3, wherein saidpneumatic closing order is obtained by bleeding of the pressurizedarcing chamber.

5. A gas blast circuit breaker as set forth in claim 1 wherein saidnormal service control means comprise an electromagnetic control valveinitiating the closing of said contact members, said emergency meanscomprising a pressure-sensitive switch actuating said electromagneticvalve when said pressure drops below said second predetermined value.

6. A gas blast circuit breaker as set forth in claim I, wherein saidnormal service control means comprise a mechanica operated memberinitiating the closing of said contact members, said emergency meanscomprising a pressure-sensitive valve cooperating with apneumomechanical converter to operate said mechanical operated memberwhen said pressure drops below said second predetermined value.

1. A gas blast circuit breaker comprising at least one arcing chambercontaining at least one pair of separable contact members, means topressurize said arcing chamber in the opencircuit and in theclosed-circuit position of said contact members, first biasing meansbiasing said contact members in their open-circuit position towardstheir closed-circuit position, second biasing means responsive to thepressure in said arcing chamber biasing said contact members continuallytoward their open-circuit position, normal service control means toselectively effect the opening and the closing of said contact members,whereby in the open-circuit position of said contact members said secondbiasing means override said first biasing means and maintain saidcontact member in their open-circuit position only when said pressure ishigher than a first predetermined value, emergency means responsive tothe pressure in said arcing chamber, said emergency means feeding apositive closing order into said normal service control means to effectclosing of said contact members when in their open-circuit position andmaintaining in the closed-circuit position of the closed contact memberswhen said pressure in said arcing chamber drops below a secondpredetermined value, said second predetermined value being higher thansaid first predetermined value.
 2. A gas blast circuit breaker as setforth in claim 1 comprising, for each pole, a plurality ofseries-connected pairs of contact members, wherein said emergency meanseffect the closing of all the pairs of contact members from theopen-circuit position when said pressure drops below said secondpredetermined valve.
 3. A gas blast circuit breaker as set forth inclaim 1, wherein said normal service control means effect a pneumaticcontrol of the movement of said contact members, said emergency meanscomprising a pressure-sensitive valve feeding a pneumatic closing orderinto said normal service control means when said pressure drops belowsaid second predetermined value.
 4. A gas blast circuit breaker as setforth in claim 3, wherein said pneumatic closing order is obtained bybleeding of the pressurized arcing chamber.
 5. A gas blast circuitbreaker as set forth in claim 1, wherein said normal service controlmeans comprise an electromagnetic control valve initiating the closingof said contact members, said emergency means comprising apressure-sensitive switch actuating said electromagnetic valve when saidpressure drops below said second predetermined value.
 6. A gas blastcircuit breaker as set forth in claim 1, wherein said normal servicecontrol means comprise a mechanical operated member initiating theclosing of said contact members, said emergency means comprising apressure-sensitive valve cooperating with a pneumomechanical converterto operate said mechanical operated member when said prEssure dropsbelow said second predetermined value.